Magnetoresistive sensors change their resistance in relation to a magnetic field. In GMR (giant magnetoresistive) sensors formed on a chip, the resistance span is about 10%. In other words when the magnetic field is directed perpendicular to the GMR resistance path in the plane of the chip, the resistance value increases by 5% compared to the resistance in the zero-field. If the magnetic field changes polarity, the resistance value drops by 5% compared to the reference value in the zero-field. For magnetic fields that are not too large, the resistance change can be approximately linearly proportional to the magnetic field acting on the chip. The constant or slope of proportionality is referred to as the magnetic sensitivity. The magnetic sensitivity may be dependent on the temperature.
On the other hand, the resistance value also increases simply by increasing the temperature: about 0.1%/° C. Some differential GMR sensor integrated circuits (ICs) contain at least two GMR resistors at a distance of about 2.5 mm on a chip, for example. The IC evaluates the difference of the fields at both positions. As a result, the sensor may become insensitive to a homogeneous background field as the sensor's noise immunity increases. Typical applications of these kinds of sensors include so-called speed GMR sensors to detect the angular velocity of a wheel for ABS (antilock braking system) or of a crankshaft or camshaft for engine controls.
Because of the large separation between the two GMR resistors, referred to as resistance strips, it is possible for the two strips to exist at slightly different temperatures. The reason for this can be due to a non-homogeneous temperature distribution outside the sensor. A temperature gradient may be generated by the sensor itself. Dissipative losses of an output stage generate thermal waves may affect the two GMRs with different time delays and different intensities.
Due to such a temperature gradient, an offset error occurs, i.e. the two resistance strips exhibit different resistances due to the temperature difference, even in the absence of a magnetic field. If, for example the temperature difference is 0.1° C., the GMR resistances will differ by 0.01%. Since the magnetic sensitivity is about 1%/mT, the magnetic field is altered by 10 μT. While this may sound small, the difference is too much if one considers that a sensor like this typically has a jitter requirement of 1 μT.
In some sensors, an H bridge that is powered from a power source may be used. Attempts are made to arrange the dominant heat sources on the chip such that the GMR resistance strips are located on the same isotherms. However, this can only be done with a moderate amount of precision since comprehensive simulations would be required, and even then there would not be enough space on the chip to place all components at ideal locations. Even with a symmetrical layout and symmetrical distribution of heat sources on the chip, asymmetric temperature distribution on the chip occurs since the lead frame is often asymmetrical and the ground pin cannot lie in the line of symmetry for 4-pin packages for example.